It is also well known that the effectiveness of an upright vacuum cleaner in collecting dust and dirt from a surface to be cleaned (called the “pick-up ratio” of the vacuum cleaner) depends in part on both the pressure difference, or “suction”, and on the airflow, as measured in volume of air moved per unit time, which are achieved at the dirty air inlet of the floorhead, although the pick-up ratio may also be improved, for example, by including a rotating brush in the floorhead to dislodge dust and dirt from the surface to be cleaned. Both the pressure difference and the airflow are themselves in turn both dependent on two things, namely the power of a source of suction which the vacuum cleaner comprises and the efficiency of the design of the vacuum cleaner in transmitting that power to the dirty air inlet of the floorhead. These relationships may best be understood by reference to the accompanying FIG. 1.
As may be seen in FIG. 1, maximum suction and therefore peak pressure difference between the dirty air inlet of the floorhead and atmospheric air is achieved when the dirty air inlet is completely occluded, whereas at this point, the airflow is of course also at a minimum. On the other hand, peak airflow through the dirty air inlet is achieved when the dirty air inlet is completely unobstructed, whereas at this point, the pressure difference drops to a minimum instead. Under normal operating conditions, the actual pressure difference and airflow will lie somewhere between these two extremes. The mathematical product of the pressure and the airflow gives a value known as the air watts, which measures the suction power of the dirty air inlet. The peak air watts are achieved somewhere between the points of peak pressure and peak airflow, at a point where this mathematical product is maximised. The efficiency of the design of the vacuum cleaner may then easily be measured as the ratio of these peak air watts achieved divided by the number of watts of electrical power supplied to the source of suction which the vacuum cleaner contains, which is typically a fan driven by an electrical motor. Thus, in order to increase the value of the peak air watts achieved, and therefore the effectiveness of the vacuum cleaner (the “pick-up ratio”), either the power of the vacuum cleaner's source of suction or the efficiency of the vacuum cleaner's design must be improved.
Increasing the power of the vacuum cleaner's source of suction has two disadvantages. Firstly, it entails increasing both the size and the weight of the source of suction. Secondly, it also increases the vacuum cleaner's power consumption. In the case of a mains powered vacuum cleaner, this has the effects of increasing the running costs and the environmental impact of the vacuum cleaner. However, in the case of a battery powered vacuum cleaner, it is particularly disadvantageous, because apart from increasing the running costs and the environmental impact of the vacuum cleaner, it also increases the size and weight of whatever battery the vacuum cleaner also comprises to supply electrical power to the source of suction. Therefore, it is more desirable to try and improve the efficiency of the vacuum cleaner's design than to increase the power of the vacuum cleaner's source of suction, and this fact is most particularly true in the case of a battery powered or cordless vacuum cleaner.
One prior art document which addresses this problem of how to improve the efficiency of design of an upright vacuum cleaner is U.S. Pat. No. 6,334,234 in the name of Conrad et al. This document describes an upright vacuum cleaner comprising a floorhead having an inlet for dirty air, an elongate body comprising a dust collection chamber and having a handle located at an upper end of said body, a duct for conveying dirty air from the inlet to the dust collection chamber, and a source of suction power for drawing dirty air from said inlet, through said duct to said dust collection chamber, wherein the dust collection chamber comprises a cyclonic separation device. According to this document, a bend in a conduit for a fluid causes a turbulent pressure loss in the conduit as the fluid travels through the bend in the conduit and the greater the sharpness of the bend, the greater the pressure loss. The pressure loss in the airflow decreases the amount of suction which can be generated at the cleaning head of the vacuum cleaner for any given motor in the vacuum cleaner and therefore the efficiency of the vacuum cleaner (column 2, lines 12 to 19). This document aims to solve this problem by positioning a motor for generating an airflow through the vacuum cleaner above the cyclonic separation device when the elongate body of the vacuum cleaner is pivoted to be generally vertical. Thus the path of clean air from the cyclonic separation device to the source of suction of which the motor is part is short and straight, and the efficiency of the upright vacuum cleaner is thereby improved.
However, it should also be mentioned in this context that the idea of placing a motor at the top of an upright vacuum cleaner above the dust collection chamber when the elongate body of the vacuum cleaner is pivoted to be generally vertical is already known from earlier European patent no. 0 439 273 B. This earlier document describes a battery-powered upright vacuum cleaner comprising a floorhead having an inlet for dirty air, an elongate body having a handle located at an upper end thereof the body housing a dust collection chamber comprising a filter bag, a duct for conveying dirty air from the inlet of the floorhead to the dust collection chamber, and a source of suction power for drawing dirty air from the inlet, through the duct to the dust collection chamber, wherein the source of suction power comprises a motor and a fan located above the dust collection chamber.
U.S. Pat. No. 6,334,234 also discloses that the upright vacuum cleaner described therein may comprise a wand having a second dirty air inlet additional to the dirty air inlet of the floorhead, the wand being for a user to perform above-floor cleaning, and a changeover valve allowing the flow of dirty air entering the dust collection chamber to be selected between the respective dirty air inlets of the floorhead and the wand, although this document gives no further details of the changeover valve, apart from stating that suitable valve means are known in the art (column 8, lines 24 to 26). Although not described in this document either, an upright vacuum cleaner made according to the teachings of this document and sold in the North American market under the Westinghouse brand, also comprises a battery for supplying electrical power to the source of suction power.
In the vacuum cleaner described in U.S. Pat. No. 6,334,234, however, the airflow pathway from the floorhead to the dust collection chamber comprises at least one sharp, right-angled bend to one side, and in some of the embodiments disclosed therein, a further bend from the duct to the inlet of the dust collection chamber, which is contrary to the teachings of this document described above that such bends should be avoided. Moreover, in the embodiment described as also comprising a changeover valve, it is not known whether this changeover valve may also introduce further contortions into the airflow pathway, thereby also affecting the efficiency of the vacuum cleaner adversely.
An object of the present invention, therefore, is to provide an improved upright vacuum cleaner, which addresses the problems inherent in the design of the vacuum cleaner described in U.S. Pat. No. 6,334,234. Another object of the present invention is to provide an upright vacuum cleaner with improved efficiency, which is particularly suitable for use with battery power.